Sampling networks may be used for a variety of uses, such as digital filtering, analog-to-digital conversion, digital-to-analog conversion, or sampling inputs with sample and hold (or track and hold) circuits. In a particular sampling network, a plurality of input signals are sampled onto an array of capacitors, sample and hold circuits, or track and hold circuits, depending on the configuration of the network. The stored signal may then be converted or filtered based on the design of the network.
Sampling networks have a number of drawbacks which do not allow for ideal operation. An example of one such drawback is distortion that is produced from various circuit elements within the network. In a sampling network that uses track and hold circuits, the input switch may be an initial source of distortion for the sampling network. This may occur in the network regardless of whether the circuit is in a track phase (where the tracking capacitors are being charged) or the circuit is in the hold phase (where the charge is held on the capacitors).
Additionally, all sampling networks, regardless of the configuration, have other sources of distortion. A particular high source of distortion in sampling networks occurs at the voltage sampled at a sampling capacitor. This is generally a result of the presence of a nonlinear resistor used in the sampling network which may be voltage dependent and thus cause distortion at the output of a sampled capacitor. Nonlinear parasitic capacitance may additionally contribute to the distortion at the sampled capacitor. One method to correct this distortion is to use a bootstrap circuit to “bootstrap” the devices in the sampling network. A bootstrap circuit that is connected to an input switch can pull up the current through the input switch and keep the gate-to-source and the back-gate-to-source voltages constant, and effectively eliminate a variation of the resistance of the nonlinear resistor from varying the input voltage. The use of a bootstrap circuit has the additional benefit of causing a reduction in a variation of a switch resistor with the input voltage.
The use of bootstrap circuits however, may lead to unwanted residual effects. For example, in a bootstrap circuit using MOSFET devices, bootstrapping a back-gate of a MOSFET device would lead to an unwanted junction capacitance at the input of the sampling network. This additional capacitance introduces nonlinearity at the input that may degrade the performance of the sampling network. Thus there remains a need in the art, for an integrated circuit which may improve linearity at the input of a sampling network, without leading to additional distortion.